Recently, in the automotive field, individual components included in exhaust gas have acted as causes of air pollution and environmental pollution, and thus tightening of regulations has been underway. Therefore, in order to decrease the amount of CO2 exhausted from automobiles and improve gas mileage, not only improvement in engine efficiencies by means of high-efficiency combustion, idling stop function, and the like and weight reduction by means of material substitution, but also improvements by means of energy source diversification due to the usage of hybrid electric vehicles (HEV), biofuel vehicles, hydrogen/fuel cell vehicles (FCV), electrical vehicles (EV), and the like have become necessary.
Regarding these requirements, efforts have also made to improve gas mileage by mounting a heat exchanger that recovers exhaust heat, that is, an exhaust heat recovery device mainly in hybrid electric vehicles. In the exhaust heat recovery device, exhaust gas heat is transferred to cooling water by means of a heat exchange, and heat energy is recovered and reused; and as a result, the temperature of the cooling water is increased. Thereby, performance for heating the inside of vehicles is improved, and gas mileage performance is improved by shortening the time required for warming up engines. The exhaust heat recovery devices are also referred to as exhaust heat recirculation systems.
In addition, other efforts to install an exhaust gas recirculation apparatus that recirculates exhaust gas have also been made. Examples of the exhaust gas recirculation apparatuses include EGR coolers. In the EGR cooler, exhaust gas from engines is cooled using engine cooling water or air, and then the cooled exhaust gas is returned to the intake side and is re-combusted. Thereby, the combustion temperature is lowered, and the amount of NOX which is harmful gas is decreased.
For heat exchange portions in the above-described exhaust heat recovery devices or EGR coolers, a favorable heat efficiency is required, and a favorable thermal conductivity is required. In addition, high corrosion resistance against exhaust gas condensate water is required because the portions come into contact with exhaust gas. Particularly, since engine cooling water flows through these parts, in the case where holes are generated due to corrosion, there may be a risk of serious accidents. In addition, materials being used have a thin sheet thickness in order to increase the heat exchange efficiency. Therefore, materials having higher corrosion resistance than that of members in the downstream portion of exhaust systems are required.
Conventionally, among members mainly including mufflers in the downstream portion of exhaust systems, for portions particularly requiring corrosion resistance, a ferritic stainless steel containing 17% or more of Cr such as SUS430LX, SUS436J1L, or SUS436L has been used. For materials of exhaust heat recovery devices or EGR coolers, corrosion resistance that is higher than or equal to that of the above-described ferritic stainless steel is required.
In addition, EGR coolers are, generally, assembled by brazing, and thus parts being used need to have high brazing properties (brazeability). Here, in order to improve brazeability, the wettability of the surfaces is important. Ti is more easily oxidized than Fe and Cr and Ti forms an oxide film with poor wettability on the surface. Therefore, the amount of Ti is desirably set to be low. Furthermore, similar to Ti, Al forms an oxide film with poor wettability on the surface. Recently, there has been a demand for a steel in which the amount of Al as well as the amount of Ti is low. In addition, since the surface roughness of a steel sheet also has a great influence on wettability, it is also extremely important to control surface properties by controlling manufacturing conditions.
In addition, when the temperature of a brazing thermal treatment is high, the temperature reaches approximately 1,200° C., and, in this high-temperature environment, crystal grains in a stainless steel grow and coarsen. Since the coarsening of crystal grains has an influence on mechanical characteristics such as thermal fatigue and the like, a stainless steel on which a brazing thermal treatment is carried out needs to have characteristics in which crystal grains do not easily coarsen even at high temperatures.
As described above, a steel that is used in the EGR coolers needs to have high corrosion resistance and favorable brazeability.
Patent Document 1 discloses an inexpensive ferritic stainless steel material which is used as muffler-constituting members or water-warming device members that form welded portions and has high corrosion resistance. This ferritic stainless steel material contains C: 0.025% or less, Si: 2% or less, Mn: 1% or less, P: 0.045% or less, S: 0.01% or less, Cr: 16% to 25%, Al: less than 0.04%, and N: 0.025% or less, and further contains one or more element selected from Ni: 1% or less, Cu: 1% or less, Mo: less than 1%, Nb: 0.5% or less. Ti: 0.4% or less, and V: 0.5% or less, with a remainder of Fe and inevitable impurities. The ferritic stainless steel material has an oxide film in which the composition of an outermost layer contains a total amount of Si and Cr of 15 atom % to 40 atom % and 5 atom % of Fe in terms of the atomic ratio including oxygen on the surface, and the composition of the outermost layer is measured by an X-ray photoelectron spectrometry (XPS).
Patent Document 2 discloses a ferritic stainless steel with high brazeability in the case where the ferritic stainless steel is brazed in an environment of a high temperature and a low oxygen partial pressure as is the case with Ni brazing and Cu brazing. This ferritic stainless steel contains C: 0.03% or less, N: 0.05% or less, C+N: 0.015% or more, Si: 0.02% to 1.5%, Mn: 0.02% to 2%, Cr: 10% to 22%, Nb: 0.03% to 1%, and Al: 0.5% or less, with a remainder of Fe and inevitable impurities. Furthermore, the ferritic stainless steel contains an amount of Ti that satisfies Expression: Ti-3N≤0.03 and Expression: 10(Ti-3N)+Al≤0.5 or further contains, as a substitute for a part of Fe, one or more of Mo: 3% or less, Ni: 3% or less, Cu: 3% or less, V: 3% or less, W: 5% or less, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% or less.
Patent Document 3 discloses a ferritic stainless steel for a automobile exhaust system member having favorable resistance to initial rusting at a low cost without impairing the intrinsic functions of automobile exhaust system members such as high-temperature strength, resistance against scale spallation, formabiliy, corrosion resistance against exhaust gas condensate water, and corrosion resistance against salt damage environments. This ferritic stainless steel contains, by mass %, C: ≤0.0100%, Si: 0.05% to 0.80%, Mn: ≤0.8%, P: ≤0.050%, S: ≤0.0030%, Cr: 11.5% to 13.5%, Ti: 0.05% to 0.50%, Al: ≤0.100%, and N: ≤0.02% with a remainder of Fe and inevitable impurities. The number of inclusions containing Ca per square millimeter of an arbitrary cross-section is less than 10, and furthermore, preferably, the proportion of the number of Mn-based sulfides to the total number of Ti-based sulfides and the Mn-based sulfides is 50% or less.
Patent Document 4 discloses a ferritic stainless steel having excellent localized corrosion resistance. This ferritic stainless steel contains, by mass %, C: 0.030% or less, N: 0.030% or less, Si: 0.30% or less, Mn: 0.30% or less, P: 0.040% or less, S: 0.020% or less, Cr: 16% to 26%. Al: 0.015% to 0.5%, Ti: 0.05% to 0.50%, Nb: 0.05% to 0.50%, and Mo: 0.5% to 3.0%, with a remainder of Fe and inevitable impurities. When the ratio of the amount of Al to the amount of Si is represented by Al/Si, the following expression (1) is satisfied.Al/Si≥0.10  (1)
Patent Document 5 discloses a ferritic stainless steel with high corrosion resistance. This ferritic stainless steel contains, by mass %, C: 0.030% or less, N: 0.030% or less, Si: 0.01% to 0.50%, Mn: 1.5% or less, P: 0.04% or less. S: 0.01% or less, Cr: 12% to 25%, Nb: 0.01% to 1.0%, V: 0.010% to 0.50%, Ti: 0.60% or less, and Al: 0.80% or less, with a remainder of Fe and inevitable impurities. The following expression (A) is satisfied, furthermore, polishing marks with an arithmetic average roughness value Ra of the surface in a range of 0.35 μm to 5.0 μm are provided, and the color difference L* value of the surface is 70 or more.0.35≤Nb+5V≤2.0  Expression (A)
However, the inventions disclosed by Patent Documents 1 to 5 are not capable of having both of excellent corrosion resistance against exhaust gas condensate water and excellent brazeability.